Recently, in wireless communication environment, coaxial cable has been used to transmit an ultra high frequency signals, because of coaxial cable's low signal attenuation characteristic.
More particularly, the coaxial cable has good characteristic, such as stabilized impedance, low signal attenuation characteristic, and excellent shielding property in high frequency band.
By reason of said high frequency characteristic, coaxial cable is suitable for high frequency communication cable which is used in base station of cellular phone using high frequency signal of microwave band.
In general, polymer material has been used as insulating material for surrounding copper wire.
In this case, if dielectric constant of insulating material decreases, then signal attenuation of cable decreases.
Therefore, developments and applications for materials of low dielectric constant have been continued in these days.
Currently, polyethylene resin has been applied for insulator, and especially, HDPE (High Density Polyethylene Resin) has been mostly used among various polyethylene resin.
On the other hand, FEP (Fluorinated Ethylene Propylene Resin) or etc. has been used for insulator among various fluororescein.
Among said materials, polyethylene resin is the best choice to make insulator of coaxial cable, because it has several advantages such as good physical properties, easy processing, and low cost.
But, by just using said materials, it is hard to transmit the signal to long distance with high speed and low signal loss rate.
Therefore, a chemical foaming method which foaming the mixture of chemical foaming agent and said materials, and a gas foaming method which foaming said materials with injected gas has been used to increase the signal transmission speed by decreasing the dielectric constant.
Here, due to the limitation of foaming skill, raising foaming rate of small size porous cell has been used to make insulator with low foaming density.
Referentially, to gain low dielectric constant, the technique of foaming a small cells which have diameter of 170 μm at most, and average diameter of the cell is from 90 to 130 μm with reduced density of foaming materials less than 0.22 g/cm3, especially from 0.17 to 0.19 g/cm3, is described in U.S. Pat. No. 6,037,545.
But, if traditional size of cell is used with extremely high foaming rate to decrease the foaming density, cell collapse or unbalanced external appearance can be occurred.
In this case, coaxial cable which produced by traditional foaming method is not suitable for long distance transmission with low signal loss, because of increased return loss of it.